Respiratory syncytial virus (RSV) is the most common cause of bronchiolitis and pneumonia in young children worldwide. An experimental RSV vaccine developed in the 1960's caused exacerbated disease in infants upon a subsequent natural infection with RSV. Recent studies using inbred mice have suggested that CD4 T cells, as well as other undefined genetic factors, contribute to RSV vaccine- enhanced.lung pathology during experimental RSV infections. This proposal outlines a two-pronged approach designed (1) to characterize the RSV-specific memory CD4 T cell response to pulmonary infection in a murine model of experimental RSV infection and (2) to identify host genetic factors that contribute to RSV vaccine- enhanced pulmonary injury. The results of these analyses will aid in our understanding of the cellular and genetic factors that mediate RSV vaccine-enhanced lung disease and provide valuable information necessary for the development of a safe and effective RSV vaccine.